1. Field of the Invention
The present invention relates to a counter.
2. Description of the Related Art
Conventional laser printers are known to have a photosensitive member, developing devices, cleaner and the like provided as units which are removable from the printer body. The aforesaid photosensitive members and the like deteriorate in accordance with an increase in the number of printed sheets. Therefore, a counter is provided to count the number of printed sheets. When the count value of said counter attains a predetermined value, the laser printer executes a warning indicating it is time to replace the unit (hereinafter referred to as "EP unit"). In general, the aforesaid counters are provided with a nonvolatile memory (e.g., E.sup.2 PROM) within the EP unit. Incrementation of the counter is accomplished when the CPU accesses the aforesaid nonvolatile memory and rewrites the count value in accordance with a count signal output from the main unit of the printer with every print.
However, about 10 .mu. seconds are required from the start of the aforesaid value rewriting to its completion when the CPU accesses the aforesaid nonvolatile memory on the EP unit side in accordance with a count signal output from the main unit of the printer. When, for whatever reason, the power source is suddenly cut off while the CPU is accessing the nonvolatile memory of the EP unit (the time between the start of writing to its completion), the present value to be counted is not incremented in the counter, such that an entirely different count value is erroneously written to memory.
In image forming apparatus such as laser printers, copying machines and the like, counters are constructed such that when continuously forming the same identical image on a plurality of sheets, the desired number of sheets for image formation is input from an input means such as a ten-key pad or the like. A countdown is executed from the aforesaid input desired number of sheets each time an image is formed on a sheet, such that when the count value reaches zero [0], the completion of image formations relative to the desired number of sheets is detected and the image forming apparatus is stopped.
In the aforesaid apparatus, when paper jams occur while continuously accomplishing image formations on a plurality of sheets, electrical noise is generated within the circuits including the aforesaid counter, which is disadvantageous inasmuch as counting errors may result.